As is known in the art, plastic couplings exist having internal metal inserts, designed to increase their strength and to avoid possible breakage when subjected to high stress; moreover, such metal inserts allow for an increase in abrasion resistance of the internal threads of the usual plastic couplings.
Such metal inserts are attached to the couplings during the actual injection molding thereof and, owing to the fact that the metal and plastic are materials having an entirely different molecular composition, a continuous joint between the two parts seldom occurs, with discontinuities being formed that lead to leakage; for this reason, a high number of couplings have to be discarded since they fail to provide the required sealing effect, thus causing heavy loss of materials and labor, and thus influencing the final cost of the part.